1. Field of the Invention
The present invention relates to an engine and a method for designing the engine and, more particularly, to such an engine capable of performing dilute combustion and improving fuel economy, while eliminating hazardous ingredients, particularly HC and NOx, within exhaust gases.
2. Description of Related Art
For engines of a spark ignition type, as represented by gasoline engines, it is considered as ideal to position ignition gaps of spark plugs, namely, sources of ignition, in the center of their combustion chamber. In other words, the spark plugs are disposed in the manner as described hereinabove from the point of view that the flame travel distance from each of the spark plugs to a periphery of the combustion chamber is set to be equal to each other in peripheral direction of the cylinder, or in a cylindrically peripheral direction.
However, recently, an improvement of fuel economy or mileage to a great extent has been required. From this viewpoint, it is considered to burn a mixed air having an air-fuel ratio sufficiently leaner than a stoichiometric air-fuel ratio, namely, to perform a so-called dilute combustion. In performing dilute combustion, sparking performance may become poor and a combustion speed may also become slow, thereby making combustion unstable.
From this point of view, as disclosed in Japanese Patent Laid-open (kokai) Publication No. 57-148,021, there is proposed rapid combustion to be performed by disposing a plurality of sources of ignition to one combustion chamber, namely, by performing a so-called multi-point ignition. Of course, from the viewpoint of performing rapid combustion, the plurality of sources of ignition are so arranged as to allow a distance between each of the sources of ignition to become as small as possible and to become uniform, namely, to allow the distance from any position of the combustion to the nearest source of ignition to become as uniform as possible.
It is to be noted, however, that rapid combustion makes a peak value of the heat production rate (a peak value of a calorific value per unit time) so large that NOx is increased rather than the advantage which should originally be achieved which is that the NOx should be reduced by making the air-fuel ratio lean. In order to reduce NOx once increased, the operation should be performed in a leaner air-fuel ratio, thereby presenting another problem in that HC is increased to a great extent. Therefore, a load region capable of performing dilute operation is, in practice, restricted to an extremely narrow range, and this is a great barrier for performing dilute combustion.